In photography, a camera flash is used to improve image picture quality in low light situations, by illuminating the scene with a burst or pulse of visible light while a picture of the scene is taken. For many portable multifunction devices or “mobile devices”, such as smart phones and cellular camera phones, a desirable choice for an integrated flash light source is the light emitting diode (LED) lamp. An LED flash has a small form factor (thereby able to fit more easily in a relatively small mobile device), and can provide continuous illumination at relatively low power (for capturing video). In a typical setting, an LED flash is pulsed on, or remains turned on, for the entire duration of an image-framing period (also referred to as a single exposure or shutter cycle for taking a picture).
In the case of an electronic rolling shutter (ERS) camera, the image framing period is skewed or stretched in time, as depicted in FIG. 2. That is because with ERS, the entire image sensor array is not reset at once, but rather staggered on a row-by-row basis. FIG. 2 depicts a continuous sequence of ERS exposures and associated flash control signal timing for two types of flash mechanisms that are currently available. For a given exposure K, a complete exposure interval (or image-framing period) starts at T1 and ends at T4. The next exposure K+1 begins at T4, whereas the previous exposure K−1 ended at T1. A typical range for the period T4-T1 may be 200-300 milliseconds. In a typical scenario, exposure begins at T1 with row 1 of the sensor array being reset and then allowed to integrate or collect photo-generated charges; this continues for the duration of a row integration time interval, ending at T3. A short delay thereafter, referred to as a row processing delay, the second row is reset and allowed to integrate. This procedure continues sequentially until the last row, row N, has been reset and allowed to integrate. The output signals from the sensors of each row may also be read or measured in a staggered manner, beginning with the first row at T3 and ending with the last row, row N, at approximately T4.
As depicted in FIG. 2, the LED flash is only turned on for the time interval T2 until T3. This time interval is referred to as the ERS row overlap interval (for a given exposure). During the ERS row overlap interval, all rows of the image sensor are integrating or collecting photo-generated charges (for capturing the scene). Note that in contrast to the LED flash, a gas discharge lamp would only need a very short pulse to illuminate the scene during the row overlap interval, due to its substantially greater light output.